1. Field of the Invention
The present invention is generally directed to the field of semiconductor devices, and, more particularly, to an SOI device with charging protection and methods of making same.
2. Description of the Related Art
In modern integrated circuits, the number and density of individual circuit elements, such as field effect transistors, is steadily increasing and, as a consequence, performance of these integrated circuits is currently improving. The increase in package density and signal performance of integrated circuits requires the reduction of critical feature sizes, such as the gate length and the channel length of field effect transistors, to minimize the chip area occupied by a single circuit element and to reduce signal propagation delay resulting from a delayed channel formation. However, currently critical feature sizes are approaching 0.1 μm and less and a further improvement in circuit performance by reducing the sizes of the transistor elements is partially offset by parasitic capacitances of the transistors formed in bulk silicon substrates.
To meet the ever-increasing demands with respect to device and circuit performance, circuit designers have proposed new device architectures. One technique to improve performance of a circuit, for example of a CMOS device, is to manufacture the circuit on a so-called silicon-on-insulator (SOI) substrate. An SOI substrate comprises an insulating layer formed on a bulk substrate, for example, a silicon substrate or glass substrate, wherein the insulating layer frequently comprises silicon dioxide and is sometimes referred to as buried oxide layer or “box” layer. Subsequently, an active silicon layer is formed on the insulating layer in which an active region for a field effect transistor device is defined by shallow trench isolation structures. A correspondingly fabricated transistor is entirely electrically isolated from the regions surrounding the transistor area. Contrary to a conventional device formed on a bulk semiconductor substrate, the isolation of the active region of the SOI device significantly suppresses parasitic effects known from conventional devices, such as latch-up and leakage currents drifting into the substrate. SOI devices are also characterized by lower parasitic capacitances compared to devices formed on a bulk semiconductor substrate and, hence, exhibit an improved high-frequency performance. Moreover, due to the significantly reduced volume of the active region, radiation-induced charge carrier generation is also remarkably reduced and renders SOI devices extremely suitable for applications in radiation-intensive environments.
However, it is also well known that, during operation, undesirable charges may accumulate below the channel region of the transistor, thereby adversely affecting the transistor characteristics, such as the threshold voltage, single-transistor-latch-up, and the like.
Furthermore, undesirable charge damage on devices may also occur during the course of fabricating such SOI structures. For example, the use of plasma-based etching processes may result in undesirable charge damage to components of the SOI device, such as the gate insulation layer.
The present invention is directed to eliminating or at least reducing the problem of undesirable charge damage during the manufacturing of SOI devices.